HUMAN BIOLOGY

Cells and how they work 51

What do endomembrane system

organelles do?

• Ribosomes on rough endoplasmic reticulum (ER) build new cell
  proteins.


• In the smooth ER and Golgi bodies, lipids are assembled and
  many proteins are modified into their final form.


• Some vesicles move substances into or around cells or transport
  them to the outside.


• The vesicles called lysosomes and peroxisomes break down
  unwanted material.

taKe-hoMe Message

In genetic conditions called lysosomal storage

diseases, an enzyme is missing from cell lysosomes.

The substance the missing enzyme would break

down builds up instead. As it accumulates, it

interferes with cell activities. Briefly research this

topic. What is the most common lysosomal storage

disease in humans? How does the disease affect a

person who is born with it?

One good information source is the Lysosomal

Storage Disease Center at the University of

California San Francisco Children’s Hospital (www

.ucsfhealth.org/clinics/lysosomal_storage_disease

_center/index.html).

ThiNk OuTsiDE ThE bOOk

to its proper destination. The pancake at the top of a Golgi

body is the organelle’s “shipping gate” for molecules to be

exported. Vesicles form there as patches of the membrane

bulge out and then break away into the cell’s cytoplasm.

Vesicles have a range of roles in cells

Vesicles may have other specialized roles in cells. An

example is the lysosome, a type of vesicle that buds

from the membranes of Golgi bodies (Figure 3.13D). A

lysosome’s function is to chemically digest (break down)

substances. It contains a potent stew of enzymes that speed

the breakdown of proteins, some lipids, complex sugars,

and nucleic acids. Lysosomes may even digest whole cells

or cell parts. Often, lysosomes fuse with other vesicles

that have formed at a cell’s plasma membrane and that

contain bacteria or other undesirable items that attach to

the plasma membrane. White blood cells of the immune

system dispose of foreign material in the vesicles.

Vesicles called peroxisomes are sacs of enzymes that

break down fatty acids and amino acids. The reactions pro-

duce hydrogen peroxide, a potentially harmful substance.

But before hydrogen peroxide can

injure the cell, another enzyme in

peroxisomes converts it to water and

oxygen or uses it to break down

alcohol. When someone drinks alco-

hol, peroxisomes of liver and kid-

ney cells are able to break down

about half of it.

budding vesicle

smooth ER channel

To cytoplasm

ribosome

A Rough ER B Smooth ER C Golgi body D A lysosome

vesicle

polypeptide

chain

Lysosome containing

ingested material

C: Biophoto Associates/Science Source; D: © Cengage Learning

lysosome Vesicle in which

enzymes break down

unwanted molecules.

peroxisome Vesicle in

which enzymes break down

fatty acids and amino acids.

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